The present invention relates to the field of calenders, and more particularly to devices for controlling the diameter of rolls used in calenders or analogous machines.
Pressing a material between two calender rolls can change the physical characteristics of the material. For example, calendering paper can change its density, thickness and surface features. Thus, the calendering process is frequently used in the manufacture of paper and other sheet materials where it is often desirable to change the density, thickness or surface features of the material.
A common problem associated with calendering is an uneven thickness of the calendered material, or "web". Localized variations in a variety of parameters affect the diameter of individual calendar rolls and create variations in the spacing or "nip" between cooperating rolls. Variations in the nip across the width of a pair of calender rolls produces a web having non-uniform thickness. Thus, a more uniform thickness could be obtained if the local diameters of the rolls could be controlled.
If the rolls are made of a material that responds to changes in temperature, one may control local roll diameters by varying the temperature of selected cylindrical sections of the calender roll. Previous devices have used this principle by directing jets of hot or cold air against sections of a rotating calender roll to control its local diameters. Many of these devices blow hot air from a supply plenum against sections of the calender roll to increase the diameter of the roll and thus decrease the thickness of the web. Alternatively, when these devices release cold air from a second supply plenum against selected cylindrical sections of the calender roll, those sections contract. This decreases the local roll diameter and increases the thickness of the web. Examples of such devices are shown in U.S. Pat. No. 4,114,528 to Walker and U.S. Pat. No. 3,770,578 to Spurrell.
These previously known devices, however, are subject to certain limitations and inefficenicies. For example, the nip control range is determined by the maximum and minimum temperatures of the air jets. The air in the hot air plenum is usually pressurized by a blower and heated by steam from the facility power plant. Typically, however, steam supplied by such a power plant is waste steam, having a maximum temperature of about 350.degree. F. Inefficiencies in the heat exchange process further limit the maximum temperature of such steam heated air to about 325.degree. F.
The calender roll control device of the present invention has a number of features which overcome many of the disadvantages of many air jet control devices heretofore known. For example, the present invention uses jets of steam to heat the calender roll. The direct use of steam avoids the inefficiencies in the air heating process. Additionally, since the invention uses steam jets rather than steam-heated air, the higher temperature provides a greater control range then conventional hot air devices. Furthermore, the invention does not require a blower to pressurize an air plenum. Instead, the steam plenum used with the present invention is pressurized directly by the thermal energy of the steam.
Another type of prior calender roll control device uses magnetic fields to heat the calender roll, for example, as shown in U.S. Pat. No. 4,384,514 to Larive et al. In this type of device, the roll is made of a conducting material and magnets are positioned close to the roll surface. As the rotating roll passes under the magnets, cylindrical sections of the roll are heated by magnetic induction. The magnetic fields induce currents in the calender roll which dissipate their energy heating the roll. However, because ordinary 50/60 Hz electromagnets have high magnetic forces which may bend the roll, 25 Khz alternating current electromagnets are generally used. Thus, effective magnetic induction calender roll control devices require a special alternating current power supply.
Furthermore, to achieve the greatest heating effect, the magnets should be positioned within about 1/8" of the roll surface. However, placing the magnets this close to the calender roll may lead to damage when the web breaks. The broken web can wrap around the roll a sufficient number of times to build up a thick layer of calendered material on the roll. Once the layer becomes more than 1/8" thick, the rotating calender roll can drive the paper into the magnets with sufficient force to damage both the magnets and their supporting structure.
The device of the present invention also provides a number of advantages over magnetic induction calender roll control devices. For example, the invention does not require a special alternating current power supply to energize electromagnets. Instead, the invention heats the calender rolls with steam which is generally a less costly form of energy then electricity. Electric power is a relatively expensive energy source since the steam to electric power conversion process is usually only about 44% efficient. The direct use of steam to heat the calender roll is more economical. Furthermore, depending upon the particular application, the steam nozzles used in the present invention to direct steam jets against the calender roll are usually positioned approximately two inches from the roll surface. This two inch gap between the nozzles and the calender roll greatly decreases the possibility of damage to the nozzle by contact with the calendered material.
The present invention thus provides a number of advantages over prior art calender roll control devices. These and other advantages will become apparent in the description which follows.